A nationwide coalition of mayors who support tougher gun control, led by Thomas M. Menino of Boston and Michael R. Bloomberg of New York, ­issued a report Monday that ­assails the National Rifle Association for what the mayors call a systematic attempt to restrict access to firearms data and research by the public, government, and law enforcement.

The report was unveiled by Bloomberg at Johns Hopkins University in Baltimore on the one-month anniversary of the shooting spree at an elementary school in Newtown, Conn., that killed 20 first-graders and six adults.

“The message of this kind of report shows why you need to keep the momentum and what we’re up against,” said Jake ­Sullivan, who is Menino’s liaison to the federal government. “You make informed choices for the health and safety of your communities, but because this is about guns, you can really see how [the NRA] has turned out the lights on our ability” to collect and share information.

The report by Mayors Against Illegal Guns, which has 800 members across the country, calls for removing federal restrictions on firearms ­research by the Centers for ­Disease Control and the National Institutes of Health, and undoing a ban that prohibits law enforcement from publicly releas­ing information on the history of guns used in crimes.

“For many Americans, this is the straw that has broken the camel’s back,” Bloomberg said of the Newtown shooting. “It’s time for Congress to stop gagging our scientists, military leaders, and law-enforcement officers and stop trying to hide the truth from the American people.”

More than 100 mayors have joined the gun control group since the Connecticut massacre on Dec. 14, Sullivan said. The coalition also released a new television ad Monday that features relatives of gun victims demanding tough action by ­political leaders in Washington.

The group is urging a broad, national focus on factors behind gun violence. Since 1996, when an NRA-backed amendment forbade the CDC from ­research that could be used to “advocate or promote gun control,” the agency’s funding for firearm-injury prevention has fallen 96 percent, according to the report. Last year, the CDC spent $100,000 of its $5.6 billion on the subject.

“The Washington gun lobby has led an aggressive effort to limit what we know about firearms. And it has largely succeeded,” the report said. “Among a group of 32 comparable nations, the United States accounts for 30 percent of the population, but 90 percent of the gun homicides.

“Despite this epidemic, the federal government conducts almost no scientific research on how criminals get and misuse guns, or what policies are effective at stopping them.”

For example, the report said, the principal research arm of the Justice Department has had its work on guns brought to a halt. Between 1993 and 1999, the National Institute of Justice provided funds for 32 gun-related studies. Under the Obama administration, the ­research agency has not funded a single public study on firearms, according to the report.

NRA-supported measures also have hobbled the efficient analysis of firearms data, the mayors said. The Bureau of ­Alcohol, Tobacco, Firearms & Explosives is barred by federal legislation from using an electronic database to organize its millions of records. Instead, the mayors reported, the agency is forced to use a paper-based system to organize the records it uses to help enforce the nation’s gun laws.

NRA president David Keene said Sunday on CNN that he ­believes Congress will not ban assault weapons similar to the rifle used in the Newtown massacre, despite President Obama’s declaration that gun control is one of his top priorities.

But Sullivan said he believes national momentum has shifted significantly toward tighter laws on firearms.

He added that he thinks top NRA officials do not reflect the views of most of their members. Independent polling of gun owners and NRA members, he said, shows that 74 percent of respondents support strong background checks.

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